Hurry it up when there are less than 3 async requests in the block io queue:

1) don't dirty throttle the current dirtier

2) wakeup the flusher for background writeout (XXX: the flusher may then abort not being aware of the underrun)

When doing 1-dd write test with dirty_bytes=1MB, it increased the XFSwriteout throughput from 5MB/s to 55MB/s and increased disk utilizationfrom ~3% to ~85%. ext4 achieves almost the same. However btrfs is notgood: it only does 1MB/s normally, with sudden rushes to 10-60MB/s.